My girl, Baby, who is about 3-4 years old has stiff joints all over her body,
The vet said we should give her vitimin suppliments for vit C which I put on her veg, it's the only was she takes it well.
It's hard to get Baby to eat enough veg with Vit C in it so they think this is the reason for her stiffness among other things from backyard breeders not looking after their stock very well.
She was also hopping when she walked and ran but that was because she had hurt her ankle joint which was made worse by the stiffness,
It sounds like your piggie could be suffering from bad nutrition in the past like your vet said.
Hopefully it's an easy fix like it was for Baby and she can start getting better!

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She had her appointment today, poor thing was sedated twice in 12 hours. They said that x-rays look normal, and the orthopedic exam went well. Just watch her and try to video tape her running.